Colors by Carl Reynolds

Home , Cadmium pigments, CIE 1931 color space

“Color is an immensely complex subject, one that draws on concepts and results from physics, physiology, psychology, art, and graphic design. The color of an object depends not only on the object itself, but also on the light source illuminating it, on the color of the surrounding area, and on the human vision system.” Computer Graphics Principles and Practice — Foley & van Dam1

We were all taught in school that the primary colors are red, yellow, and blue. This statement implies, all colors can be created from mixing various amounts of red, yellow, and/or blue. This is wrong. Red, yellow, and blue are not the primary colors. Neither are red, green, and blue; or cyan, magenta, and yellow. Each of these is a set of primary colors, but no ﬁnite set of colors can be used to create all the colors visible to the human eye. It might be better to call these a color space, instead of primary colors. It is possible to create all the colors within the red-yellow-blue space, by using various amounts of red, yellow, and/or blue, but there are colors outside this space that are visible to the human eye.

Figure 1.

Depending on what you’re doing, you might want to use one, or another color space. For example, most painters use red, yellow, and blue, along with black, and white. If you’re printing color magazines, or books you’ll probably use cyan, magenta, yellow, and black to do your designs. Many computer graphics artists design their work using red, green, and blue. If you’re creating textiles, you will probably use red, yellow, and

1 J. Foley, A. van Dam, S. Feiner, J. Hughes. Computer Graphics Principles and Practice, pages 563-585. Addison-Wesley, second edition, 1990.

Almost any three colors can serve as primary colors, depending on how you want to use them. The only relevant issues are (1) the actual range of mixtures (gamut) you are able make with the colorants, and (2) whether this gamut produces the desired visual effect in the images you want to represent. — Bruce MacEvoy 2

What do we mean by all colors? We must recognize that there are colors outside the range of the human eye. A spectrum of “all colors” might look something like Figure 1. Note that there are no non-spectral colors, such as magenta, and that colors in the infrared and ultraviolet ranges, not visible to most humans, are bands of black.

The range of wavelengths3 for Figure 1 extends far outside the normal human range. Depending on the species, butterﬂies have ﬁve or six types of color receptors4. Mantis shrimp, with 16 types of receptors, can see deep into the ultraviolet. Sparrows with four types of color receptors see into the extreme infrared, and perceive many more greens and blues than humans.

When younger I could see several different bands of color on both sides of the spectrum into the ultraviolet and infrared. As I’ve aged, I can no longer see these extra colors, leading me to believe that all of us can see different ranges of color, making each of us unique in our abilities to see various colors, not only from each other, but even from ourselves at different times of our lives.

In the 1850s while doing color matching experiments James Clerk Maxwell noticed large individual differences in color perception among his test subjects, leading to physiological complexities in the measurement of color vision. These arise from the prereceptoral ﬁltering of blue and violet light by the lens and macular pigment, and from individual differences in cone sensitivity and photopigments, and in the proportional numbers of long, medium and short cones5 in the retina.

2 http://www.handprint.com/HP/WCL/color6.html 3 My preference is to refer to the frequency of various colors. My second choice would be to measure the wavelength of light in angstroms (10-10 meters), but most color scientists measure light by its wavelength in nanometers so I will use the standard abbreviation ‘nm’ when referring to light measurements in this article. 4 Humans normally have three types of color receptors. I use ‘normally’ in the mathematical sense. There are some people who have limited or no use of some of the three receptor types (we refer to these people as “colorblind”). There may be people who have the use of four color receptors. However, on a Gaussian distribution, normal is to have adequate use of three color receptors. 5 The terms ‘long’, ‘medium’, and ‘short’ refer to the wavelengths of the light that the three visual receptors in the human eye are most sensitive to. ‘Long’ receptors are sensitive to wave lengths from about 480 nanometers to about 740nm. Their peak sensitivity is around 575nm. The medium receptors have a peak sensitivity around 545nm, and the short receptors peak sensitivity is around 435nm.

ColorsCarl Reynolds © 2014Page 2 Ancient Color Usage History shows that humans have always been attracted to color. We have found paintings in red, orange, dark purple, brown, orange, and dark green using powdered earths and plants on cave walls up to 40,000 years old.6,7 The Ancient Egyptians from as early as 2650b.c.8 and the ancient Greeks from c. 530b.c. created paintings on vases, walls, and panels using powdered earth, and glass to make glazes and stucco. The Greeks used white, black, red, orange, and some green9. Ancient Egyptians are the only culture before the Renaissance Figure 2. known to use blue. Their colors included white, black, red, orange, yellow, green, and purple10. The Romans used dry painting techniques, and frescos to create a large variety of paintings11.

Tempera paints are made by adding pigments to prepared egg yolks. The egg dries very quickly and the consistency of the color achieved can vary from batch to batch so it must be used quickly12,13. You don’t want to mix too much at a time because of spoilage, but you want to mix enough to cover the desired area because of difficulty mixing a second batch to match the first. Tempera and water color paints had been used from the time of the Ancient Egyptians, possibly even earlier14. While tempera was widely used until the Renaissance, water colors were only used for sketches, and manuscript illustrations until Albrecht Dürer (1471–1528), started using it to create a number fine wildlife, and landscape paintings in the German Renaissance15.

Although the use of oil paints may have been discovered much earlier in other cultures, most Renaissance sources credit the Flemish artist Jan van Eyck (c.1390-c.1441) with

6 http://en.wikipedia.org/wiki/Cave_painting 7 http://wiki.answers.com/Q/What_did_aboriginal_people_use_to_paint_with 8 http://www.crystalinks.com/egyptart.html 9 http://www.essential-humanities.net/western-art/painting/greek 10 http://www.crystalinks.com/egyptart.html 11 http://en.wikipedia.org/wiki/Roman_art 12 http://www.renaissanceconnection.org/lesson_science_egg.html 13 http://www.renaissanceconnection.org/lesson_art_oil.html 14 http://en.wikipedia.org/wiki/Tempera 15 http://en.wikipedia.org/wiki/Watercolor_painting

ColorsCarl Reynolds © 2014Page 3 the invention of oil paints16. In the 1470s an oil painting by Hugo van der Goes arrived in Florence17 and is thought to have inﬂuenced Leonardo da Vinci (1452-1519) to become the ﬁrst painter in Florence to use oil paints.

We believe the Ancient Greeks arranged colors in the following order: black, red, green, yellow, white (from dark to light). From his notes, we know that one of the ways Leonardo da Vinci arranged color was white, yellow, brown, red, green, and blue18 (from light to dark). This scheme allowed him to improve on the technique of drawing and painting called chiaroscuro, and expand its use in the art world. Da Vinci also worked with palettes such as red, Figure 3. yellow, blue, green, black, and white, calling these the simple colors, but, he acknowledged that greens and blues were actually complex colors because they could be created by combining other colors19. It is possible that we see Leonardo da Vinci on the cusp of a time when people were shifting from the use of one organization of colors (arranged from dark to light) to another (arranged in color mixing order).

Chalks, and glazes are not conducive to being mixed; when you try, you get a mixture of particles, but the colors themselves don’t mix. Because water colors and tempera paints dry so quickly, attempts to mix their colors usually result in a muddy-grey mess. Because of the nature of these media, it’s unlikely that much work was done with color mixing before the introduction of oil paints.

It is probable that the introduction of oil paints allowed people, for the ﬁrst time, to successfully mix paints and see the creation of a different color. Oil may have been the spark that started the interest in answering questions such as “What colors can be used to make another color?”; and “Is there a minimal set of colors that can be used to make all the other colors?”. Keep in mind that the range of colors from which to select a set of primary colors was limited by the colors of oil paints available. Today, we might pick cyan, magenta, and yellow as primary colors (as printers using inks did in the eighteenth century), but, in the 1500s cyan was thought of as a combination of blue and white, and magenta didn’t exist until 185920. These are probably factors that may have led artists of the sixteenth century to select red, blue, and yellow as primary colors.

16 http://en.wikipedia.org/wiki/Oil_painting 17 http://simple.wikipedia.org/wiki/Oil_paint 18 http://www.coloracademy.co.uk/ColorAcademy%202006/palettesandmixing/historical/crenaissance/ erenaissance/davinci.htm 19 http://arthistory.answers.com/renaissance/how-leonardo-da-vinci-inﬂuenced-color-theory 20 http://en.wikipedia.org/wiki/Battle_of_Magenta

ColorsCarl Reynolds © 2014Page 4 How We See Colors English Prime Minister William Gladstone21 (1809-1898) was trained as a classicist. He was particularly fond of Homer and studied his works in detail. One of the things Gladstone noticed was Homer’s odd use of color for descriptions. Homer referred to “the wine dark sea”, faces green with fear, wine colored oxen, violet sheep, etc. In his book Studies on Homer and the Homeric Age 22 (1858), Gladstone performed an extensive analysis of language related to the use of color in the Iliad, and the Odyssey. Gladstone found a total lack of the use of blue, and minimal use of red, yellow, and green. Searching other Ancient Greek texts he found similar results, and decided that the Ancient Greeks must have been colorblind. ... This conclusion was probably not a correct.

Ten years later Lazarus Geiger23 (1829–1870) conducted a much larger survey of ancient texts in many languages including Icelandic, Chinese, Norse, German, Hebrew, etc. He found results similar to Gladstone’s. He analyzed over 10,000 lines of Vedic poetry which describe sunsets in vivid detail, the stars, the moon and the sky, yet there is no mention of blue in any of these.

Geiger posited that you don’t learn to acknowledge a color until you have a name for it, and you won’t have a name for the color until you can make the color for yourself. He came to the conclusion that people gradually acquire color over time and in a particular order24.

Later, Berlin and Kay 25 (1969), modiﬁed Geiger’s order of color acquisitions to the following list. 1. All languages contain terms for dark(cool), and light(warm) (e.g, black, and white) 2. If a language contains three terms for colors, it contains red. 3. If a language contains four terms for colors, it contains green, or yellow. 4. If a language contains ﬁve terms for colors, it contains green, and yellow. 5. If a language contains six terms for colors, it contains blue. 6. If a language contains seven terms for colors, it contains brown. 7. If a language contains eight or more terms for colors, it contains purple, pink, orange, and/or grey. 8. A language will add divisions for tints of green, or blue only after purple, pink, orange, and grey have all been added. 9. Words distinguishing intensity appear only relatively late in this process.26

21 http://en.wikipedia.org/wiki/William_Ewart_Gladstone 22 http://en.wikipedia.org/wiki/Studies_on_Homer_and_the_Homeric_Age 23 http://en.wikipedia.org/wiki/Lazarus_Geiger 24 http://www.radiolab.org/story/211213-sky-isnt-blue/ 25 http://en.wikipedia.org/wiki/Basic_Color_Terms:_Their_Universality_and_Evolution 26 http://www.handprint.com/HP/WCL/color2.html#language

ColorsCarl Reynolds © 2014Page 5 A number of authors, such as Barbara Saunders (1995), John Lucy (1997), Stephen C. Levinson (2000), Anna Wierzbicka (2006), and Nicola J. Pitchford & Kathy T Mullen (2006), have expressed opposition to Berlin and Kay’s ideas of the gradual revelation of color terms. However, some studies, such as Lenneberg&Roberts, and Brown&Lenneberg, have shown support for Berlin and Kay’s hypothesis.27

Although there are still no deﬁnitive theories of how we learn to recognize color and the relation of language to color, a number of researchers are ﬁnding some very interesting results in this area. Dr. Anna Franklin and her team at the University of Surrey, Gilford, England28 have determined that children who have not developed language abilities process colors on the right sides of their brains, but once they develop language, color processing moves to the left side where the major language centers are.29

Jules Davidoff, and his research assistant Serge Caparos, both of Goldsmiths University of London, England, have studied the use of color terms among the Himba tribes of Namibia, a people who only have six words for various colors. They refer to Figure 4. the sky as black, and water as white. They have a color name that encompasses some greens, reds, and browns, but unlike many western languages do not have separate words for green and blue. Davidoff and Caparos performing tests with color chips found that the Himba could easily distinguish a brownish-yellowish-green chip from other green chips (a test most westerners would fail), but had difﬁculty distinguishing blues, and bluish-greens from other green chips (a test most westerners would easily pass). This kind of test has been performed with many other cultures around the world showing when their language doesn’t have words to distinguish between green and blue, the people have difﬁculty distinguishing these colors.

27 http://en.wikipedia.org/wiki/Linguistic_relativity_and_the_color_naming_debate 28 http://www.sussex.ac.uk/psychology/people/peoplelists/person/256540 29 http://www.boreme.com/posting.php?id=30670#.UvGfPP1PLG4

ColorsCarl Reynolds © 2014Page 6 Figure 4 shows two of the tests Davidoff used with the Himba. Most Himba can immediately see the different sample in the upper circle, but have difﬁculty seeing the different one in the lower circle. Most westerners are the opposite.

In Japan, because the name for green was only introduced a little over a century ago, the people will often refer to items we would call “green” as “blue”30. For example, the Japanese will often refer to the green light in a trafﬁc signal, as blue. There are many more examples of this in Japan, and other cultures31.

Development of Color Science With the arrival of oil paints in the sixteenth century making it easier to experiment with color mixing, people began to consider the question, “How do we see color?”. Renowned scientists, mathematicians, and natural philosophers such as Isaac Newton32 (1642-1726), Gottfried Wilhelm von Leibniz33 (1646-1716), and Johann Wolfgang von Goethe34 (1749-1832) considered this question and came to various conclusions as to the causes.

Early chemists, motivated by the commercial importance of textiles, studied dyes and pigments to improve them. The Irish chemist Robert Boyle35 (1627-1691) wrote in 1664 that the painter's "simple and primary colors" were black, white, red, yellow and blue, which could "imitate the hues (though not always the splendor) of those almost numberless differing colors" found in nature.

In June 1661 Newton was accepted to trinity College, Cambridge to read Physics, Mathematics, and Astronomy. He received his Bachelor of Science degree in January 1665. In August 1665 when the students and faculty were sent away as a precaution against Black Plague, Newton went home. He spent the time away from school thinking about optics and gravity, which led to his invention of the Calculus.

One of his important experiments in optics was to sit in a darkened room with a glass prism, and when a beam of light came through a hole he had made in the window shade, he placed the prism in the beam. He observed that the white light from the sun was split into the spectrum of colors of the rainbow. Next, to determine if the colors in the split light were coming from the white light, or were being introduced by the prism, he placed a second prism in the blue part of the spectrum from the ﬁrst prism. When the second prism did not add more colors to the blue light, he concluded that the white light contained all the colors of the visible spectrum, and the prism was separating these colors into their individual wavelengths. This was a very difﬁcult idea for people of that

30 http://www.wired.com/wiredscience/2012/06/the-crayola-ﬁcation-of-the-world-how-we-gave-colors- names-and-it-messed-with-our-brains-part-i/ 31 http://en.wikipedia.org/wiki/Distinguishing_blue_from_green_in_language 32 http://en.wikipedia.org/wiki/Early_life_of_Isaac_Newton 33 Leibniz 34 http://en.wikipedia.org/wiki/Johann_Wolfgang_von_Goethe 35 http://www.bbk.ac.uk/boyle/boyle_learn/boyle_introduction.htm

In 1704, Newton was the ﬁrst person to arrange the colors in a circle36. He took the bar of colors from the spectrum and transformed it into a segmented circle, where the size of each segment differed according to his calculations of its wavelength and of its corresponding width in the spectrum. The sections of the wheel are labeled (in Latin) Rubeus, Aureus, Flavus, Viridis, Caeruleus, Indicus, Violaceus, arranged in the order red, orange, yellow, green, blue, indigo, and violet37. The placement and size of the colored sections of Newton's circle suggested several mathematical and harmonic relationships, such as complementary colors, and additive color mixing. Figure 5. The big question about color at the time was whether it was an inherent property of light, or a result of the way it acted once it enters the eye. Newton’s observations were considered by the scientiﬁc community to prove conclusively that colors are contained in light.

Leibniz was a rival of Isaac Newton, and independently of Newton discovered the Calculus. In fact, the English will tell you it was invented by Newton, while the French and Germans will tell you it was Leibniz. He believed that the colors we see are the result of interactions of light within our bodies. He spent much of his time dissecting the eyes of cows, pigs, and sheep to try to prove this theory.

Jacob Christoph Le Blon38,39 (1667-1741) was a German painter and engraver who, inspired by reading Newton’s Opticks, invented a system using three separate printing plates, each inked with one of the painter's primary colors red, yellow, or blue (and sometimes a fourth plate inked with black) to create full color mezzotint prints — the practical basis for today's multicolor process printing. Le Blon published a document in Dutch in 1707 using the four-color process he had invented. In 1717 he moved to London, then to Paris in 1735, teaching his color printing techniques, and using it to

36 http://www.webexhibits.org/colorart/bh.html 37 http://www.colourlovers.com/blog/2008/05/08/history-of-the-color-wheel/ 38 http://en.wikipedia.org/wiki/Jacob_Christoph_Le_Blon 39 http://www.coloracademy.co.uk/ColorAcademy%202006/palettesandmixing/historical/baroque/rococo/ leblon.htm

ColorsCarl Reynolds © 2014Page 8 publish a number of pamphlets and books in both places. He was granted a patent for the three-color process while in England. In the late nineteenth and early to mid- twentieth century some commercial printing, continued to use of his red-yellow-blue technology, even though the more versatile cyan-yellow-magenta, and cyan-yellow- magenta-key processes had been adopted by most printers. In these processes, cyan is sometimes referred to as "process blue"; magenta is sometimes called "process red". The key color, is usually black.

In 1793, Benjamin Thompson, Count Rumford40 (1753–1814), coined the term "complementary color". After discovering that colored lights and their shadows had perfectly contrasting colors, he wrote, "To every color, without exception, whatever may be its hue or shade, or however it may be compounded, there is another in perfect harmony to it, which is its complement, and may be said to be its companion."

Additive Colors Subtractive Colors Primary Colors red, green, blue red, blue, yellow Secondary Colors cyan, magenta, yellow orange, green, purple red⬌cyan, red⬌green, Complementaries green⬌magenta, blue⬌orange, blue⬌yellow yellow⬌purple Table 2. In 1802 Thomas Young41 (1773–1829), showed it is possible to create most of the colors of the spectrum, including white light, by mixing various intensities of red 42 , green, and blue lights. He showed that it is possible to create magenta by combining red and blue; yellow by adding red and green; and cyan by adding green to blue. Young, based on his experiments with light, was the ﬁrst to postulate that the human eye contains three types of color receptors. This led Hermann von Helmholtz43 (1821-1894) to develop the Young-Helmholtz theory44 of trichromatic vision in 185045.

In the early 1800s David Brewster46 (1781–1868), the inventor of the kaleidoscope, proposed a theory competing with Young’s that the true primary colors were red, yellow,

40 http://en.wikipedia.org/wiki/Benjamin_Thompson 41 http://en.wikipedia.org/wiki/Thomas_Young_(scientist) 42 Until the time of Young, the color called “red” in the red-yellow-blue color model had been more blue than the color we call “red” today,. When Young was experimenting with lights, the color he used for red looked more yellow than people were used to and is sometimes called “red-orange” instead of “red”. 43 http://en.wikipedia.org/wiki/Hermann_von_Helmholtz 44 http://en.wikipedia.org/wiki/Young–Helmholtz_theory 45 http://www.colorsystem.com/?page_id=812&lang=en 46 http://en.wikipedia.org/wiki/David_Brewster

In the early nineteenth century, scientists and philosophers across Europe were studying the nature and interaction of colors. Goethe stated that the two primary colors were those in the greatest opposition to each other, yellow and blue, representing light and darkness. He wrote, "Yellow is a light which has been dampened by darkness; blue is a darkness weakened by light." He explained that red comes from the opposition of blue and yellow, through a process called "augmenta- tion".

Goethe was vehemently opposed to Newton's analytic treatment of color, engaging instead in compiling a com- Figure 6. prehensive rational description of a wide variety of color phenomena. In the late eighteenth century, Goethe noticed that if he stared at a bunch of daffodils, when he looked away the image of the daffodils would persist, but in purple rather than yellow (the ‘negative after image’ effect). With this as evidence, he resurrected the debate about whether color was the result of processes outside or within the eye.

Goethe modiﬁed Newton’s color wheel by removing indigo. This placed each color opposite it’s visual complement on the circle. For example, red is opposite green, blue opposite orange, etc. Figure 6 is an example Goethe’s color wheel.

In 1810, Goethe published his Theory of Colours, which he considered his most important work. It contains some of the earliest published descriptions of phenomena such as colored shadows, refraction, and chromatic aberration. In it, he characterized color as arising from the dynamic interplay of light and darkness through the mediation of a turbid medium. Through Schopenhauer’s 1816 On Vision and Colors and Charles Eastlake translation into English in 1840, Goethe’s theory became widely adopted by the world of artists, notably the Pre-Raphaelites, Turner, and Kandinsky. Even though Goethe’s theory eventually lost favor with the scientific community because of a lack of scientific rigor, he was the first to systematically study the physiological effects of color, and his observations on the effect of opposed colors led him to a symmetric arrangement of his color wheel, “for the colors diametrically opposed to each other … are those which reciprocally evoke each other in the eye.”

ColorsCarl Reynolds © 2014Page 10 In 1839 Eugene Chevreul published a book showing how complementary colors can be used for choosing colors for everything from textiles to garden ﬂowers. In 1867 French art critic Charles Blanc, and later American color theorist Ogden Rood47 (1831-1902) each published books further popularizing the use of complementary colors. All these books were enthusiastically read by artists in America, England, France, and Germany, especially such artists as Georges Seurat, and Vincent van Gough. They had a paradigm changing inﬂuence on all the artists of the nineteenth century. Claude Monet wrote, “Color makes its impact from contrasts rather than from its inherent qualities … the primary colors seem more brilliant when they are in contrast with their complementary colors”. Orange and blue became an important combination for all the Impressionist painters.

Most of the advances in color science during the nineteenth century, and well into the twentieth century, were related to color photography, and color printing. In the 1850s, James Clerk Maxwell was the first to show that operations with colored lights (additive colors), and those with pigments (subtractive colors) use different rules. He demonstrated that additive and subtractive color spaces use different colors for primary, secondary, and complementary colors. He also realized that a finite set of primaries cannot be used to create all possible colors. Maxwell's work on color is considered to be the basis for modern colorimetry and his work with Thomas Sutton led to the development of color Figure 7. photography. In 1931 the Commission Internationale de l'Éclairage (CIE) created the first mathematically defined color spaces, CIE-RGB and CIE-XYZ. They were derived from experiments done in the 1920s by William David Wright and John Guild. Their experimental results were combined into the specification of the CIE-RGB color space, from which CIE-XYZ was derived.

Figure 7 is an example of a CIE chromaticity curve.

47 http://en.wikipedia.org/wiki/Ogden_Rood

ColorsCarl Reynolds © 2014Page 11 Color Spaces All color models use one or more of the following design requirements: 1. A color specification defining all possible colors as a mixture of fundamental attributes, such as "primary" colors 2. A geometrical framework locating all colors with respect to each other, and to the fundamental attributes 3. Standardized, unique color labels 4. Pigment mixture recipes or physical color examples that can be used to match the abstract color specification to natural or manufactured objects48

Most color spaces are defined by a finite set of primary colors, for example, the red- green-blue color space, or the cyan-magenta-yellow-black color space. However, there are some, such as the CIE color spaces that are mathematically defined, and are not bounded by a finite set of colors.

The colors that deﬁne the color space are called the primary colors. Primary colors are the set of colors that can be combined to make a useful range of colors. Usually the primaries cannot be created by combining other colors within the color space.

The secondary colors can each be made by adding equal amounts of two of the primaries, for example, in the red-green-blue color space the secondary yellow can be made by adding red and green, cyan by adding blue and green, and magenta by adding red and blue.

You can make tertiary colors by adding equal amounts of any of the secondaries with either of its adjacent primary.

Complementary colors are pairs of colors which, when combined in the right proportions, produce a neutral grey. Usually complementary colors are of the same saturation and lightness (or value). In some color spaces, complimentary colors will not combine to make a neutral color, so another way to deﬁne complementary colors in these spaces is to select two colors on diametrically opposing sides of the color space’s color wheel. The pairs of complementary colors vary depending upon the color model, and how the color is made.

Red-Yellow-Blue Color Space Red-Yellow-Blue is the color space used by people who use oil, water, and acrylic, paints. It is use by people who make or use face makeup, dye textiles, or paint houses portraits or landscapes. This is a subtractive color space.

It should also be noted that, while most of the people who work in this color space think of it as the red-yellow-blue color space, there are very few people who only have three colors in their working palette. For example, a painter may have Naples Yellow, Cadmium Red, Burnt Sienna, Hansa Yellow, Alizarin Crimson, Phthalo Blue, Phthalo Green, Yellow Ochre, Ivory Black, and Titanium White in their paint box, and may even

48 http://www.handprint.com/HP/WCL/color6.html

A color wheel for this space is shown in Figure 6. This is the color wheel proposed by Goethe in his opposition to Newton’s color theory. The primary colors for this space are red, yellow, and blue. The secondary colors are orange, green, and purple. Orange is red plus yellow; green is yellow plus blue; purple is red plus blue.

You can create tints of any of these colors by adding white to it. For example, you can make pink by adding white to red, or a light orange by adding white to orange. You can also create shades of any color by adding black to it. Maybe white and black should be included as primaries in this color space.

The complementary colors are red⬌green, yellow⬌purple, and blue⬌orange. Note that the complementary colors in the red-yellow-blue color space are same ones we see when we experience the ‘negative after image’ effect. This is not true for the other color spaces, and to date, no one has found a good explanation of this difference. The ‘negative after image’ effect is the set of colors you see if you stare at a color for about a minute, then suddenly switch to look at a white space.

Unfortunately, mixing two different hues of paint reduces saturation and typically makes the mixture darker. Mixing with white changes both saturation and lightness while mixing with black only affects lightness. These qualities make it difﬁcult to know, without practice, which pigments to mix to create a desired color.

Even though many, through the nineteenth century, and even today, continue to teach that red, yellow, and blue are “the primary colors” from which you can mix all other colors, they are not the best of primary colors. They produce magentas, and purples very poorly, and they don't produce very pure greens. There are many colors in the gamut of human color vision they cannot produce at all.

Red-Green-Blue Color Space In 1802 Thomas Young, performing experiments with colored light from candles, created an understanding of the red-green-blue color space. He Figure 8. postulated the existence of three types of photoreceptors in the human eye, each of which was sensitive to a

ColorsCarl Reynolds © 2014Page 13 particular range of visible light. In 1850, his experiments led Hermann von Helmholtz to further develop the theory that the three types of cone photoreceptors could be classiﬁed as short- preferring, middle-preferring, and long- preferring according to their response to the wavelengths of light. The relative strengths of the signals detected by the three types of cones are interpreted by the brain as color. This is called the Young-Helmholtz theory, in honor of the two men who devised it.

From 1855 to 1872, James Clerk Max- well49 (1831-1879) published a series of articles concerning the perception of color, colorblindness, and color theory, resulting in his receiving the Rumford Medal50 for On the Theory of Colour Vision. In his 1855 article, Maxwell proposed that three black-and-white photographs could be taken of a scene, using a red, a green, and a blue filter respectively. The transparent prints from these photographs could be superimposed on a screen, using three projectors, each using a filter of the same color the transparency was taken with, producing a color image of the scene. Figure 9. In 1861, Thomas Sutton51 (1819-1875), inventor of the single-lens reflex camera, was the first to put Maxwell’s theory of color photography into practice. He took a series of pictures using different colored filters, as Maxwell had suggested. Then he and Maxwell were able to project a color image from these plates. Unfortunately the emulsions available to Sutton were primarily sensitive to blue, much less sensitive to green, and almost totally insensitive to red, so the results of the experiment were disappointing, but it eventually led to the development of color photography.

49 http://en.wikipedia.org/wiki/James_Clerk_Maxwell 50 http://en.wikipedia.org/wiki/Rumford_Medal 51 http://en.wikipedia.org/wiki/Thomas_Sutton_(photographer)

ColorsCarl Reynolds © 2014Page 14 The red-green-blue color space is used in the creation of color photographs, the creation and viewing of color motion picture ﬁlm, color slides, color television, and color computer moni- tors. Note that even though our modern televisions display colors in red-green- blue space, the signal is encoded in a very different space for transmission.

This an additive color space. The primary colors for this space are red, green, and blue. The secondaries are cyan, magenta, and yellow. The complementaries are red⬌cyan, green⬌magenta, and blue⬌yellow. A red-green-blue color wheel is shown in Figure 8. Figure 10. If you project overlapping lights of red, green, and blue onto a white wall, you will see a color pattern similar to Figure 10. Note that the red and green lights overlap to make yellow, the green and blue lights make cyan, and the blue and red lights make magenta. Also, magenta is opposite green in this arrangement, yellow is opposite blue, and cyan opposite red, showing that the red-green-blue and cyan-magenta-yellow color spaces are each others complements.

If you display varying intensities of red, green, and blue on the axes of a Cartesian coordinate system you create a color cube representing all the colors in both the red- green-blue, and cyan-magenta-yellow color spaces. Note that the origin will be black, and the antipodal corner will be white. The edge opposite red will show saturations of cyan, opposite the green edge will be magenta, and opposite the blue edge will be yellow. See Figure 9 for examples of a color cube.

Cyan-Magenta-Yellow Color Spaces In 1707 Jacob Christoph Le Blon invented the four-color process for printing color images. He used metal plates with differing intaglio patterns to print color images with red, yellow, blue, and black paints. This was in essence a spot color printing process since the paints were not allowed to overlap and mix on the print.

For close to 150 years, the technology of color printing remained essentially unchanged from that invented by Le Blon. By the mid 1800s inks had evolved from red, yellow, and blue to process-red (magenta), process-blue (cyan), and yellow. These new colors were more transparent than the older colors. Their transparency allowed one ink to be printed on top of another ink to create a different color, a technique that could not be used with the older, more opaque colors.

ColorsCarl Reynolds © 2014Page 15 In 1850, William Fox Talbot52 (1800- 1877) invented halftone printing53. A series of photographs of an image is taken using different color ﬁlters through a set of equidistant lines scratched into a glass plate. The lines ﬁlter the areas of color in the image into dots that vary in size proportional to the intensity of the color in that area.

If a screened photograph is taken with a red filter, cyan light will be stopped by the filter, and magenta and yellow light will reach the film. The areas of the image that reflect cyan will be darkest on the film and those that reflect magenta and yellow will be lightest. We can then invert the transparency which Figure 11. will give a plate for printing cyan.

When this process is completed for each of the red, green, and blue filters, we will have respectively a color plate for printing cyan, magenta, and yellow inks. The glass screen for each color will rotated at a different angle so that we get less interference between the different inks, and reduce occurrence of moiré patterns. The page will be printed first using the lightest color (yellow), then cyan, magenta, and finally black, again creating less interference between inks54,55,56.

Because of the nature of the inks, printing yellow, cyan, and magenta together in the same area to produce black actually produce a muddy grey, and the entire image appears muted. People learned that they could remove some of the cyan, magenta, and yellow areas from their respective color plates and add a fourth plate which printed black ink in those areas. This saved on the amount of ink and gave a much greater dynamic range to the colors in the printed picture.

All these developments in color printing led to the creation of the ﬁrst four-color rotary printing press at the Chicago Inter-Ocean (newspaper) in May 1892.

52 http://en.wikipedia.org/wiki/William_Fox_Talbot 53 http://en.wikipedia.org/wiki/Halftone 54 http://en.wikipedia.org/wiki/Color_printing 55 http://en.wikipedia.org/wiki/Color_photography 56 http://en.wikipedia.org/wiki/Carbon_print

ColorsCarl Reynolds © 2014Page 16 There are several reasons for using black ink along with cyan, magenta, and yellow57: 1. The "black" generated by mixing cyan, magenta and yellow inks is usually a muddy grey. They don’t produce black, or nicely desaturated grey. Four-color printing uses black ink to make up for this deficiency. 2. Text is typically printed in black and includes fine detail (such as character stems, and serifs). Trying to line up three passes of ink to produce text, or finely detailed outlines can be next to impossible, and often produces a blurry result. Using black to print text and fine line detail eliminates this registration problem. 3. Printing an area with three passes of ink causes the paper in that area to become soaked with ink. This often leads to bleeding of the inks out of the desired area, and can lead to unacceptable drying times, as well as buckling of the paper. 4. Using black ink is less expensive because you only need one pass of ink instead of three, thus using less time and less ink. 5. Adding black to cyan, magenta, and yellow creates a much greater dynamic range of colors, so you get deeper shadows, and more vivid highlights than you can with cyan, magenta, and yellow alone.58

This is a subtractive color space. The primary colors are cyan, magenta, and yellow. The secondary colors are red, green, and blue. The complementary colors are cyan⬌red, magenta⬌green, and yellow⬌blue. A color wheel for the cyan-magenta- yellow color space is shown in Figure 8. Note that this is the same color wheel created for the red-green-blue color space. They are the same because the red-green-blue color space is the complement of the cyan-magenta-yellow color space and will result in the same color wheel, with a possible change in rotation.

If you use cyan, magenta, and yellow inks to create areas of overlapping color on a piece of paper, as in Figure 11, you will see a common image used to display the relationship between the primary and secondary colors of cyan-magenta-yellow color space. Note that magenta and yellow combine to make red, cyan and yellow combine to make green, magenta and cyan combine to make blue, and all three together make black.

HSL, HSV Color Spaces When using the red-green-blue, or cyan-magenta-yellow color space, knowing how to produce a desired color can be very difﬁcult. For example, in red-green-blue, if you’re currently using a medium cyan and want to change to a darker cyan, you would need to decrease the amount of red, and increase the amount of blue and green. Figuring out how to make this kind of change, or which combinations of red, green, and blue to use for creating tan, pink, brown, or myriad other non-spectral colors, can be very complicated, and counterintuitive.

To create a more intuitive color model, scientists at Xerox, Palo Alto Research Center (PARC), and the New York Institute of Technology (NYIT) independently developed

57 http://en.wikipedia.org/wiki/CMYK_color_model 58 http://shutha.org/node/815

ColorsCarl Reynolds © 2014Page 17 similar color models. In the August 1978 issue of Computer Graphics, George Joblove and Donald Greenberg of PARC published an article describing their HSL color model. In the same issue Alvy Ray Smith of NYIT published an article describing his HSV model.59

These color models are used primarily by computer programs such as Photoshop®, or Painter®. It is much easier to specify a color such as pink, brown, or banana yellow using HSL, or HSV than it is to specify the same color using red- green-blue, cyan-magenta-yellow, or cyan-magenta-yellow-black values. Another advantage is that conversion between red-green-blue, and HSV or HSL is very fast to compute, so it can be done in real time. In 1979 Tektronix introduced a color graphics terminal which used HSL natively.

In both models the ‘H’ means hue. Hue is represented on a color circle using red, orange, yellow, green, cyan, blue, purple, magenta and any visible color between these. Red is at 0°, yellow is 60°, green 120°, cyan 180°, blue 240°, and magenta 300°.

In both models the ‘S’ means saturation60. The saturation of a color is determined by a combination of light intensity and how much it is distributed across the spectrum. The Figure 12. most saturated color is achieved by using just one wavelength at a high intensity. If the intensity drops, or if more wavelengths are added, the saturation drops.

As you move from the outer edge of the color circle in a straight line toward the center, the resulting color will have less of the dominant hue, and will become less saturated.

59 http://en.wikipedia.org/wiki/HSL_and_HSV 60 Note that the same color will have the same hue in both HSV and HSL spaces, but (possibly) different saturations.

In the HSL model, ‘L’ (for lightness) deﬁnes the amount of white, grey, or black added to the dominant hue to create a tint or a shade. The central (lightness) axis of the HSL color cones runs from black at 0% to white at 100%, with grey in the middle at 50%. You use the saturation to determine the amount of hue added to the white, grey, or black. The HSL color model satisﬁes the condition that mixing a color with white will lighten the color without changing its chromaticity, and similarly adding black will darken without changing chromaticity.

In the HSV color model, the ‘V’ means value. In some other color models, the term value means the same thing as lightness, but not in HSV color space. In HSV, you specify a hue and saturation, then the value indicates the amount of black you add to the color. When the saturation is 0% the value axis ranges from white to black, but when saturation is greater than 0%, the value speciﬁes the amount you want to darken the color. In HSV color space adding white to a color will lighten, and change its saturation, thus changing its chromaticity.

Both these color spaces are actually cylindrical. Most of the time they are represented as conical in shape because the hues tend to fade out as the value, or lightness approach their limits. For example, while it is valid to specify an HSL color with (0%, 100%, 0%), the lightness overrides the hue (red) and saturation so that the color you see is black. This is true for many of the colors approaching the saturation and lightness limits of display-ability, and is the reason the HSL model is often shown as a double cone, and the HSV model as an inverted cone. Figure 12 shows the double cone shape of the HSL color space, and the single cone shape of the HSV color space.

People often use HSV, HSB, HSL, HSI, and HLS interchangeably. They are not the same. There are subtle differences in the deﬁnitions of value, brightness, lightness, and intensity. The deﬁnitions of saturation used by HSV and HSL differ dramatically. It is not unusual to see people using chroma and saturation as if they were equivalent. All of these terms have subtle mathematical differences which make a difference to color scientists, but are beyond the scope of this article. For a more in-depth introduction to terms such as hue, radiance, luminance, lumina, brightness, colorfulness, chroma, and saturation, see the article on HSL and HSV at Wikipedia61, and Computer Graphics by Foley and vanDam62

One of the disadvantages of the HSL and HSV color spaces is that each unique red- green-blue device has a unique associated HSL and HSV model. This means that an image created with one of these spaces may appear different when moved to a different

61 http://en.wikipedia.org/wiki/HSL_and_HSV#Color-making_attributes 62 J. Foley, A. van Dam, S. Feiner, J. Hughes. Computer Graphics Principles and Practice, pages 563-585. Addison- Wesley, second edition, 1990.

ColorsCarl Reynolds © 2014Page 19 device63. Both of these representations are used widely in computer graphics, and one or the other of them is often more convenient than red-green-blue for selecting colors for image creation, but both are also criticized for not adequately separating color- making attributes, and for their lack of perceptual uniformity.

Note that there are no primary colors in these color spaces. We start with a continuous circle of hues and then use saturation, and value or lightness to modify the dominant hue. Sometimes people will start the circle with the six colors red, yellow, green, cyan, blue, and magenta, but these are just convenient points to specify the complete circle of hues.

Because we start with a continuous circle of hues, there are no secondaries in these color spaces. You could start with red, green, and blue on the color circle, then add cyan, magenta, and yellow as secondaries, but you could just as easily start with cyan, magenta, and yellow and go the other way. These are additive color spaces.

Once we have specified a given color with hue, saturation, and value/lightness, we can find it’s complement by finding the color directly across the circle from the specified color. People have also used the color circle in these color models to define a number of mechanisms for generation of color schemes including monochrome colors, analogous colors, complementaries, split complementaries, trichromatic colors, and tetra-chromatic colors.64 Excellent tools for seeing color combinations using different color schemes can be found at Work with Color - Tools, and Color Scheme Designer.

The monochromatic color scheme uses variations in lightness and saturation of a single hue. This scheme looks clean and elegant. Monochromatic colors go well together, and the scheme is very easy on the eyes, especially with blue or green hues. You can use it to establish an overall mood. The primary color can be integrated with neutral colors such as black, white, or gray. However, it can be difﬁcult, when using this scheme, to highlight the most important elements.

The monochromatic scheme is easy to manage, and always looks balanced and visually appealing. However, it lacks color contrast. It is not as vibrant as other schemes. For best results, use tints, shades, and tones of the key color to enhance the scheme.

The analogous color scheme uses colors that are near each other on the color wheel. They share similar hues and saturations. One color is used as a dominant color while others are used to enrich the scheme. The analogous scheme is similar to the monochromatic scheme, but offers more nuance while retaining it’s simplicity and elegance.

63 This statement assumes the image is stored in the color space it was created with. In a program like Photoshop® where an image is stored using CIE-L*a*b* color space, no matter what color space is used when creating it, the shift in color due to moving the image to a different device is reduced. 64http://www.color-wheel-pro.com/color-schemes.html

ColorsCarl Reynolds © 2014Page 20 The analogous scheme is as easy to create as the monochromatic, but looks richer. However, it lacks color contrast. It is not as vibrant as the complementary scheme. For best effect, avoid using too many hues, because this may ruin the harmony. Also, avoid combining warm, and cool colors in this scheme.

The complementary color scheme is made of two colors that are directly opposite each other on the color wheel. As pointed out by Count Rumford in 1793, complementary colors are intrinsically high-contrast and draw attention. This scheme looks best when you put a warm color against a cool color. Using one color for the background and its complementary color to highlight important elements, you will get color dominance combined with sharp color contrast.

The complementary scheme offers stronger contrast than any other scheme. However, it is harder to balance than monochromatic and analogous schemes, especially when desaturated warm colors are used. For best results, place cool colors against warm ones. If you use a warm color as an accent, you can desaturate the opposite cool colors to put more emphasis on the warm colors. Avoid using desaturated warm colors.

The split complementary color scheme is a variation of the standard complementary scheme, but offers more variety. It associates a color with the two colors near its complement. This provides high contrast without the strong tension of the complementary scheme.

The split complementary scheme offers more nuances than the complementary scheme while retaining strong visual contrast. However, it is harder to balance than the monochromatic and analogous color schemes. For best results, use a single warm color against a range of cool colors to put an emphasis on the warm color. Avoid using desaturated warm colors.

The trichromatic color scheme was originally defined as three colors equally spaced around the color wheel. Note, however, that Color Scheme Designer has redefined the trichromatic colors to encompass both the split complementary and trichromatic colors, allowing you to select the main color and two additional colors with 10°…180° of separation from each other. This allows more flexibility, but for esthetics it might have been better to stop the split at 120° of separation, as defined by the original trichromatic scheme.

This scheme is popular among artists because it offers strong visual contrast while retaining balance, and color richness. The trichromatic scheme doesn’t provide as much contrast as the complementary scheme, but it looks more balanced and harmonious. For best effect, use one color in larger amounts than the other two. If a combination looks gaudy, try using a combination of less saturated colors.

ColorsCarl Reynolds © 2014Page 21 The tetra-chromatic color scheme65 is the richest of all the schemes because it uses four colors arranged into two complementary pairs. This scheme is hard to harmonize.

The tetra-chromatic scheme offers more color variety than any other scheme, but, is the hardest scheme to balance. For best results, avoid using highly saturated colors in equal amounts. If the scheme looks unbalanced, try using less of, or a less saturated version of one or more colors.

Color Scheme Designer also has a color scheme they call an accented analogous color scheme. It is a combination of the analogous and complementary schemes. Like analogous, it allows you to pick a color and a set of other colors near it, but, it also adds the complement of the dominant color. This color scheme gives the richness of the analogous color scheme, and overcomes its inability to create contrast by adding the complement. I have not been able to ﬁnd anyone else using this term, although it does seem to ﬁll a need.

For best effect with the accented analogous scheme, avoid using too many hues. Use warm colors for the analogous part and cool colors to for the complement to draw the most attention to parts of your image. Avoid using desaturated warm colors as the complementary part of the scheme.

65 Sometimes called the double complementary color scheme.

ColorsCarl Reynolds © 2014Page 22 Color Matching Systems Starting with Francis Glisson66 (c. 1597-1677), a number of attempts have been made to create color matching systems to allow people to use various kinds of color scales (often composed of color chips or swatches) to match the colors of objects around them with a standardized color scale.

Glisson created an accurate gray scale by combining various amounts of white lead and lamp black to create a scale of 24 different levels of grey. Along with similar scales for red, Figure 13. yellow, and blue, he believed that any color could be matched against these scales. He was a physician and thought his scale would be a good tool for matching the colors of skin, eyes, and hair, and as a diagnostic tool for judging the colors of healthy, or diseased body parts.

In the early 1850s, James Clerk Maxwell invented the color top, a wooden disc with a series of colored circular paper wedges that could be fastened to it. When the top is spun, the colors of the paper discs combine to create a new color. With this top Maxwell was able to perform a series of experiments through which he studied human color response. He demonstrated that all colors could be synthesized by different combinations of the three primary lights: red, green and blue. A good description of how to make your own color top can be found at Bruce MacEvoy’s Guide to Watercolor Painting.

Maxwell was not completely satisfied with the results of his experiments with the color top and created a device for mixing colored lights through a system of prisms, mirrors and neutral filters enclosed in a long, flat box. This device, called a Maxwell Box, became what we today call a colorimeter. These devices and Maxwell’s reports on his color matching experiments with many people led to the modern science of colorimetry.67

Using his color box, Maxwell discovered that some pigments were more saturated than any mixtures of his three primaries could match. For example, gamboge is a more intense yellow than any mixture of light primaries. Rather than adding additional primaries to his color model to cover these additional colors, Maxwell subtracted chroma from the target colors. He did this by mixing the target color with grays with

66 http://www.whonamedit.com/doctor.cfm/2329.html 67 http://rsta.royalsocietypublishing.org/content/366/1871/1685.full?sid=1c09aaff-5e91-4bc8- a513-083e89d555e2

The amount of desaturating color required to make this match was used to estimate how far the chroma of the target color was outside the triangle made by the gamut of the primaries. This was a crucial step in the development of color science, because primary colors no longer had to be real colors. Maxwell’s method of color subtraction was extended and explained by the American physicist Ogden Rood, who showed that it permitted accurate measurements of pigment chroma even if the target color is more intense than the additive primaries used in the analysis.68

In 1853, Hermann Grassmann69 (1809-1877) proposed a set of laws that describe mathematically how colors are matched, and how they are mixed. These laws are the basis of all mathematical procedures used in the science of colorimetry.70

The work of Glisson, Grassmann, and especially Maxwell laid the foundations of color science which led Munsell to create his color model in the 1890s, led to the creation of the CIE color standards of 1931, and led to the creation of the Natural Color System in 1979.

Munsell Color Model In the 1890s, Albert H. Munsell71 (1858-1918), a teacher at the Massachusetts Normal Art School, conceived of a new system for classifying, matching, and naming colors. Inspired by the study of music, he wanted to describe color based on its three- dimensional attributes of hue, value and chroma. Because Munsell wanted to base his color model on equally perceived differences in each attribute, he found that his color space could not be a geometrically regular model.

Munsell learned to use Maxwell’s color top, and to create perceptually equal scales of gray or color chroma from Ogden Rood. The use of subjects viewing the color top to create the color space makes it a partitive color space, as opposed to being additive, or subtractive. Although the color top itself is additive. The Munsell Color Model is the first color model to use color attributes in three dimensions, one for each attribute. Working with Rood’s techniques for a decade, Munsell was finally able to publish the basic principles for his color model in his 1905 book, A Color Notation. In 1915, he published The Munsell Atlas of Color, a fifteen page compendium of color samples, which has led to the Munsell color model of today. In the 1920s, the US National Bureau of Standards became interested in his color model, leading to the adoption of the Munsell model 1943 as a color standard.72

68 http://www.handprint.com/HP/WCL/color6.html 69 http://en.wikipedia.org/wiki/Hermann_Grassmann 70 http://spie.org/x32967.xml 71 http://en.wikipedia.org/wiki/Albert_Henry_Munsell 72 http://munsell.com/about-munsell-color/development-of-the-munsell-color-order-system/

ColorsCarl Reynolds © 2014Page 24 [The Munsell Color Model] is recognized as a standard system of color specification in standard Z138.2 of the American National Standards Institute, Japanese Industrial Standard for Color JIS Z 8721, the German Standard Color System DIN 6164, and several British national standards. The Munsell color-order system has been widely used in many fields of color science, most notably as a model of uniformity for colorimetric spaces and has, itself, been the subject of many scientific studies. — Munsell Color Company

At the center of the Munsell Color Model is a column with black at the bottom, and white at the top. This column indicates the value of a color. Black, white, and the grays between them contain no hue, and are called “neutral colors”. Colors that have a hue are called “chromatic colors.” The value scale applies to chromatic as well as neutral colors and range from 0, for pure black, to 10, for pure white, with decimal divisions for each, resulting in 100 steps on the value scale. See Figure 14.

At the middle of the value column is a circle of hues. The Munsell Color Model uses the “principal hues” red, yellow, green, blue, and purple, and allows you Figure 14. to mix adjacent colors to obtain a continuous variation from one to the other. Munsell arranged the “principal hues” at equal intervals around a circle, and inserted ﬁve intermediate hues: yellow-red, green-yellow, blue-green, purple-blue and red-purple. To simplify the notation, he used R, YR, Y, GY, G, BG, B, PB, P, and RP, for the color names. A number greater than 0.00 and less than or equal to 10.00 precedes the hue designator. ‘5’ indicates the pure, or central hue. For example, 5R indicates a pure red, 10R is a hue half way between red and yellow-red, 7.5R is one quarter of the way from red to yellow-red. You can see from the callout at the bottom of Figure 14 that a monotonic scale from 5R to 5Y would be 5R, 7.5R, 10R, 2.5RY, 5RY, 7.5RY, 10RY, 2.5Y, 5Y. With ten decimal divisions between each color, Munsell has divide the hue circle into 100 equal parts.

Chroma describes the amount of a pure hue added to a color to differentiate it from the neutral color of the same value. Colors of low chroma are sometimes called “weak,” while those of high chroma are said to be “highly saturated,” “strong,” or “vivid.” The scaling of chroma is intended to be visually uniform and is very nearly so. The scale starts at zero, for neutral colors, but there is no arbitrary end to the amount of chroma.

ColorsCarl Reynolds © 2014Page 25 Figure 15 shows a representation of the irregularity of the Munsell color space caused by the various amounts of chroma for each hue and value. As new pigments have become available, Munsell color chips of higher chroma have been made for many hues and values. The chroma scale for normal reﬂecting materials extends beyond 20 in some cases. Fluorescent materials may have chromas as high as 30.

Color is speciﬁed with hue-value/ chroma. Hue is speciﬁed with a number and one of the color codes listed above. An example color would be 7.5P 6.5/7. Figure 15. This color is 25% of the way between Purple and Red-Purple. It is 15% more white than a neutral purple, and it contains 70% of the pure hue. Figure 16 shows the hue-value/chroma chips for two complementary hues from the Munsell color space.

The Munsell colors are standardized as carefully prepared color samples or paint chips, presented on separate pages of a reference catalog, the Munsell Book of Color. Each page contains color samples of a single hue, arranged in a two dimensional grid deﬁned by value and chroma. A target color is identiﬁed by placing it next to the atlas samples until the nearest color match is found.

While the Munsell Color Model acknowledges complementary colors, they do not combine to a neutral grey. Complementaries are those colors that are 180° across the color wheel from each other.

In 1935 the Optical Society of America spectrophotometrically measured the 1929 Book of Color and published the CIE-xyY values in 1940. This revealed major discrepancies between the 1915 and 1929 color samples, and major irregularities in the spacing of color samples on the CIE-xyY chromaticity diagram in the chroma intervals but also in the lines of equal hue. In this way the Munsell model and the CIE model helped to improve each other’s accuracy. In 1943, a new set of 2100 Munsell color samples was published, along with a corresponding set of CIE-xyY values, as a mathematical formula to calculate Munsell value from CIE-XYZ tristimulus Figure 16. values.

ColorsCarl Reynolds © 2014Page 26 Natural Color System® In the 1930s Tryggve Johansson proposed a new color matching system called the Swedish Natural Color System®, based on the opponent color theory Ewald Hering published in his 1874 work Das natürliche System der Farbempﬁndungen (The Natural System for Color Perception). In 1952 Professor Sven Hesselgren73 (1907-1993) using and enhancing Johansson’s research, created a color atlas, which was considered essential for implementation of the Natural Color System®. In 1979 the ﬁrst edition of the Natural Color Figure 17. System® color atlas, with 1412 colors, was launched as a Swedish national standard (SIS). The standard was the result of 100 man years of color research covering Psychology, Physics, Chemistry, Color Science, Architecture and Design. It has become the color design and communication tool for the majority of architects and designers in Europe and has been adopted as a color standard in many European and African countries.

The Natural Color System® is represented in 19 countries and has been adopted as a standard for color designation in Sweden (since 1979), Norway (since 1984) and Spain (since 1994). It is also one of the standards used by the International Colour Authority, a leading publisher of color trend forecasts for the interior design and textile markets.74

Natural Color System® starts with a color wheel with red and green on opposite sides, and yellow and blue at ninety degrees to them. There are one hundred divisions between each color. Colors are deﬁned by values specifying the amount of blackness (darkness), chromaticity, and a percentage value between two opposing hues. The amount of white in a color can be calculated by subtracting the sum of darkness and chromaticity from 100%. Achromatic tones are indicated simply with the proportion of black, followed by N for neutral.75

‘NCS S 2070-Y60R’ describes a color that is included in the standard collection (S) and lies in between yellow (Y) and red (R). The color is composed of 60% red, 40% yellow, 20% blackness, 70% chromaticity, and white is 100% - (70% + 20%) = 10%. Look at Figure 17 to see where this color would reside in the NCS color space.

‘NCS S 1500-N’ describes a light gray containing 15% black, and 85% white.

The Natural Color System® is a partitive color space, based on the opposition of colors.

73 http://sv.wikipedia.org/wiki/Sven_Hesselgren 74 http://en.wikipedia.org/wiki/Natural_Color_System 75 http://www.ncscolour.com/en/ncs/how-ncs-works/logic-behind-the-system/

ColorsCarl Reynolds © 2014Page 27 CIE-Color Spaces In the 1850s and 1860s the well known Scottish Physicist, James Clerk Maxwell, performed many experiments with color and created the device that led to the colorimeter of today. Arthur König76 (1856-1901), a student of, and later colleague of Herman von Helmholtz, used Maxwell’s Box to collect luminosity data and published his ﬁndings in 1891.77 Research on Figure 18. luminosity data continued into the 1920s primarily at American research institutions such as Bell Telephone Laboratories, the U.S. National Bureau of Standards, and the National Electric Lamp Association.

In 1922 Leonard Troland78 (1889-1932) stated in the Report of the Committee on Colorimetry for 1920-1921 in the Journal of the Optical Society of America, that the Illuminating Engineering Society of America, and the Optical Society of America had individually adopted as a standard a luminosity function on a 10nm interval from 380nm to 780nm, based on P.G. Nutting’s recalculations to König’s original data. Troland’s data was adopted as the standard 1924 luminosity function, V(λ) (where λ stands for wavelength), by the Commission Internationale de l'Éclairage.

In 1929 W. David Wright (1908-1998) published a set of three luminosity functions derived from his color matching experiments at Imperial College. Earlier John Guild, working independently at the National Physical Laboratory, had experimentally derived a similar set of functions, but had not published them. These two sets of data agreed so well that they were adopted in 1931 as the basis of the CIE-RGB color space. See Figure 18 for a graphical representation of these values.

On September 18, 1931, John Guild (1889-1979) opened a meeting of the Colorimetry Committee of the CIE, convened at Trinity College, Cambridge, England. He presented ﬁve resolutions to the Committee which would prove to be the most important single event in the ﬁeld of colorimetry, because they would set the agenda for all colorists for

76 http://en.wikipedia.org/wiki/Arthur_K%C3%B6nig 77 Visual Color and Color Mixture: The Fundamental Color Space, Jozef Cohen, (pp152-153), © 2001, University of Illinois Press 78 http://en.wikipedia.org/wiki/Leonard_T._Troland

ColorsCarl Reynolds © 2014Page 28 the foreseeable future. A fascinating, detailed history of this meeting can be found in How the CIE 1931 Color-Matching Functions Were Derived from Wright–Guild Data79.

Here are the implications of the ﬁve resolutions presented by John Guild: 1. They accepted as implicit the validity of Grassmann’s Laws. 2. The 1924 luminosity function was used as the basis of R, G, B luminance factors at 700nm, 546.1nm, and 435.8nm, with ȳ being set equal to the 1924 V(λ). 3. All values of the luminance functions were transformed so all values are positive. 4. The units of the luminance functions are chosen so the area under each curve is the same. 5. The chromaticities of the reference stimuli would be chosen so the projection of the luminosity diagram would have the maximum area possible. 6. Deane B. Judd80 (1900-1972) proposed setting the long-wavelength end of the z̄- function to all zeroes, Thus reducing the number of calculations from three factors to two for the long-wave region.81

Because all calculations at that time were carried out by hand, several considerations were given to reducing the possibility of error, such as making sure that all values were positive, and reducing the number of factors needed for a solution in certain areas of the functions. The original luminosity functions derived by Wright and Guild contained negative values for some of the values for each luminosity curve. These curves are called the CIE-RGB Color space.

By normalizing these values and applying the five proposals made by Guild, the observational errors of the previous models could be removed making the CIE-XYZ Color space the first color space based solely on mathematical calculations. For the model to include all the colors visible to the human eye, and the enclosing triangle joining the three primaries of the XYZ Color space to pass just slightly outside, but not too far from, the spectral locus in the RGB chromaticity diagram, the X, Y, and Z primaries must all be imaginary colors, that is, colors not visible to the human eye. The CIE-XYZ color space is the first color model to use imaginary, rather than real colors to define the space, and to be based solely on a mathematical model, rather than observed colors.

[I]t is noteworthy that the delegates who formulated the 1931 resolutions exhibited an admirable readiness to submerge their individual priorities for the sake of reaching agreement by consensus. — Fairman, Brill, and Hemmendinger

79 How the CIE 1931 Color-Matching Functions Were Derived from Wright–Guild Data, Hugh S. Fairman, Michael H. Brill, and Henry Hemmendinger, © 1996 80 http://en.wikipedia.org/wiki/Deane_B._Judd 81 How the CIE 1931 Color-Matching Functions Were Derived from Wright–Guild Data, Hugh S. Fairman, Michael H. Brill, Henry Hemmendinger, © 1996.

ColorsCarl Reynolds © 2014Page 29 The mathematical description of the CIE-XYZ Color space is called the 1931 Standard Observer. This means that the chromaticity diagram describes what would be seen by an average, or standard person looking at the color spectrum.

Maxwell’s method of color matching was to ask a subject to vary the intensities of three overlapping source lights to create a spot that matched the standard white target. For each experiment, the hue of one of the source lights could be varied, allowing the experimenter to measure the variation in intensities needed to match the target. Color Figure 19. matching experiments in the late nineteenth, and early twentieth centuries showed that Maxwell’s method of matching against a white target tends to produce errors in the resulting values, especially when matching against colors with high saturation, or lights with a single wavelength.

Williams, and Guild used an alternate method with three lights of maximum saturation (called “primaries”). They used targets of various colors composed of either single, or multiple wavelengths. The subject changed the intensity of three single wavelength sources to match the color of the target. The Computer Science and Engineering class on Science and Art of Digital Photography at the University of Washington, Seattle created an excellent example, in Flash, of this kind of color matching experiment.

I know that at this point you’re thinking what fun it would be to work through the mathematics showing the derivation of our modern color spaces such as CIE-LUV, and CIE-L*a*b* from the Williams and Guild data, and I agree with you. Unfortunately, such a derivation is beyond the scope of this paper. Perhaps it can be the subject of a later paper going into the mathematical details of all CIE color spaces, but for now we must be satisﬁed with an overview of the properties and advantages of the CIE color space.

Figure 18 Shows a graph of the color matching functions as derived from experiments by Guild and Williams. Note that at some places each of the graphs dip below the x-axis indicating a negative amount of that wavelength was added to the source colors to match the target color for that wavelength. The mathematical implication of this is that to match targets of certain wavelengths, it is necessary to shoot light out of the eye of the observer. In actuality, it was necessary to add an amount of that primary to the target color so a match could be made.

If we plot these three luminosity function on a 3-dimensional coordinate system, one axis for each color attribute, we will get a graph similar to the one in Figure 19.

ColorsCarl Reynolds © 2014Page 30 Projecting this three dimensional curve onto a constant luminosity plane yields the a line called the spectral locus, which combined with the line of purples creates the familiar CIE-XYZ chromaticity diagram in Figure 20.

The straight line connecting two colors on the chromaticity diagram deﬁnes all the colors that can be mixed with them (note: since the chromaticity diagram was derived from matching colors with lights, mixing two subtractive colors on the chromaticity diagram will not result in a straight line). Picking any three colors on the diagram creates a triangle showing all colors that can be mixed Figure 20. with various amounts of the three. In fact, you are not restricted to a triangle. You may select any convex polygon, and the area within these points will subtend all the possible colors mixable by the colors at the points of the polygon. The points that deﬁne one of the color areas are called the color gamut for those colors.

Figure 21 shows the extent of red-green-blue (green line), cyan-magenta-yellow-black (black line), and red-yellow-blue (yellow line) color spaces in CIE-Luv. These ﬁgures make it obvious that red-green-blue, and cyan-magenta-yellow-black have larger gamuts than the traditional red-yellow-blue space. You can also see why you loose saturation, and dynamic range of color when creating an image in red-green-blue color space, then printing in cyan-magenta-yellow. There are many colors in the red-green- blue color space that will need to be collapsed when converting an image to cyan-magenta-yellow-black.

Because of the way the Colorimetry Committee deﬁned the primaries for the CIE-XYZ color space, all visible colors must lie within the chromaticity diagram. This implies that all visible colors can be deﬁned by mixing various amounts of the X, Y, and Z primaries.

The position of the “white point” on a chromaticity diagram can be adjusted to deﬁne the brightness of the light source allowing the Standard Observer to adapt to a wide range of situations, providing a great deal of ﬂexibility to the areas the chromaticity diagram can be Figure 21.

Terms such as hue, brightness, lightness, and chroma can be deﬁned using the chromaticity diagram, giving us a more precise means of establishing each. See do "primary" colors exist? for more details of these deﬁnitions.

There have been several modiﬁcations to the CIE-XYZ color space since 1931. CIE- UCS (1960), which has been replaced by CIE-UCS (1976), is mostly used to calculate correlated color temperature. CIE-UWV (1964) is useful for measurements over a larger ﬁeld of view than CIE-XYZ. Due to its linear addition properties, CIE-LUV (1976) is useful for additive mixtures of lights. The main purpose of CIE-L*a*b* (1976) is to produce a color space that is more perceptually linear than other color spaces. Perceptually linear means that a change of an amount in a color attribute should produce a change of about the same visual importance if another color attribute is changed by the same amount.

The CIE Color Spaces show that is possible to create a mathematical model based on three values that closely approximates the response of the human eye to color. While not perfect in it’s reproduction of what we see, it is close enough to be useful in a wide range of applications and ﬁelds of interest. The CIE Color Spaces are ﬂexible enough that it is possible to map them between many other color spaces such as RBG, CYMK, Munsell, and the Natural Color System. For example, approximately 10 units of CIE- L*a*b* lightness is equivalent to 1 unit of Munsell value, and 10 units of CIE-L*a*b* chroma is equivalent to 2 units of Munsell chroma.

In the twentieth century, with increased interest in creating various color models, it became evident that none of the existing color models provided a uniform means of predicting which colors would be produced from various colors across the color space, or of calculating uniform color differences between colors across the color space. The ability to measure a perceived color difference is very important to such diverse areas as description of photographic dyes, governmental standards, textiles, food products, automobiles, etc. As a consequence, in 1947 the Optical Society of America in collab- oration with the US National Bureau of Standards began an effort to design a color space with uniform perceived color differences. Their goal was to create a color space in which the Euclidean distance between any two colors in the space would have same perceived difference in color as any other two colors that were the same distance apart. They used a rhombohedral lattice as the underlying structure of their color space. Unfortunately, by 1967, they concluded that it is not possible to represent uniform color differences in a three dimensional color model. Any color model that represents colors using only three dimensions (such as hue-saturation-value, red-green-blue, or CIE-XYZ) cannot represent color relationships uniformly.

When thinking about color systems, it is important to keep in mind that primary colors are either imaginary, invisible, and can produce all visible colors; or they are real, visible, and produce a limited gamut of colors. All choices of primary colors are arbitrary.

82 CIELab Color Space, Gernot Hoffmann, http://docs-hoffmann.de/cielab03022003.pdf

ColorsCarl Reynolds © 2014Page 32 If they are imaginary they are chosen to make measurements and calculations easier. Selection of “real” primaries depends on convenience of color mixing calculations, cost and availability of color ingredients, and color interactions with the image medium.

Color Naming When I was a boy I learned about naval navigation. One of the things we learned as part of that was a procedure called “boxing the compass”. To “box the compass” you start at north and name all the intermediate points around the compass. For example, “north”, “north by east”, “north-northeast”, “northeast by north”, “northeast”, etc. You can continue this pattern all the way around the compass, yielding 32 points on the compass.83

When I started learning about color spaces, I wanted to create a system of naming colors similar to “boxing the compass”. There are a number of problems with trying to create a color Figure 22. naming system similar to the one used on the compass, not the least of which is that colors are not evenly distributed around the color circle, and we know from earlier discussions that it’s not possible to create any color space in three dimensions so that all color differences in the space can be represented so they are perceptually uniform.

On the other hand, Munsell and the Natural Color System have tried to make models that were perceptually uniform, and over small color ranges they were moderately successful. These color models have been very useful in establishing standards useful for describing and discriminating colors. The problem with these color systems is that their notation is not intuitive to the average person.

For personal use, we usually don’t need an arithmetically accurate color system. Also, designating colors by a series of letters and number can be confusing and counterintuitive. We prefer a system of names for colors, much as we prefer to use names instead of numbers for people, months, days of the week, etc.

If we start with a traditional set of colors such as “red”, “orange”, “yellow”, “green”, “blue”, and “purple”, we have six of commonly used names for colors. “red”, “green”, and “blue” are each one syllable words which can be easily modiﬁed to be used as adjectives, such as “reddish-orange”, or “greeny-yellow”.

83 http://en.wikipedia.org/wiki/Boxing_the_compass

ColorsCarl Reynolds © 2014Page 33 Starting with red, green, and blue, in an additive color space we can use combinations of these to add “cyan”, “magenta”, and “yellow”. Even though “cyan”, and “magenta” have been in reasonably common usage for about 150 years, they are not well deﬁned, as many people’s idea of cyan can vary from light or sky blue, to a fairly dark bluish- green, and magenta can vary from bluish-purple to a bright rosey-red. Also the words themselves, “cyan”, and “magenta” are not easily modiﬁed. “cyany”, and “cyanish” do not slide easily off the tongue, and “magentay”, and “magentaish” are even worse. However, cyan and magenta are commonly enough used when speaking of color, that we’ll go ahead and use them.

If we deﬁne yellow as being between red and green, cyan between green and blue, and magenta between blue and red, we get the following ordered list of six colors: red, yellow, green, cyan, blue, and magenta.

If we then try to create a list of tertiary colors we start running into problems with our choices for color names. Red plus yellow gives us orange. Blue plus magenta yields purple. The other color names are much harder to ﬁnd. We want names with one or two syllables and that can be easily modiﬁed. Chartreuse, for example is not easy to say to begin with, and becomes almost impossible when we try to say “chartreusy”, or “chartreusish”.

Here is an ordered list of color names I’ve found that could be used to define an evenly spaced set of colors on the color circle: red, orange, yellow, ___, green, viridian, cyan, azure, blue, purple, magenta, and rose. I’m not very happy with “viridian”, “azure”, and “rose”, and notice there is no useful name for a yellowish-green color. Viridian, and azure are difficult to say, and do not accept modifiers such as “-ish”, or “-y” well. Rose is a name we often use to describe a shade of pink, so using it to describe a hue between magenta and red is changing its common usage. I will use the name “jaune” to describe the color midway between yellow and green. This is French for yellow, making it a bad choice since it already has a meaning decidedly different from the one we are using, but for the purpose of this paper, I’ll use it for the time being. I’m open to suggestions for other names to use for these color names.

Next we divide the spaces between each of these twelve colors by using a set of quantitative adjectives to define different amounts of each color. Faintly, slightly, significantly, and extremely. For example, • Red • Faintly Orangish-Red • Slightly Orangish-Red • Orangish-Red • Significantly Orangish-Red • Extremely Orangish-Red • Extremely Reddish-Orange • Significantly Reddish-Orange • Reddish-Orange • Slightly Reddish-Orange • Faintly Reddish-Orange • Orange

ColorsCarl Reynolds © 2014Page 34 We can create an ordered list of names to describe the chroma of a color: vivid, strong, pastel, mild, and neutral. We can then use a set of modiﬁers for these chroma names similar to the ones used for the hues: noticeably, considerably, somewhat, lightly, and marginally. Thus the set of tints for red would be • vivid, red • noticeably vivid, red • considerably vivid, red • somewhat vivid, red • lightly vivid, red • marginally vivid, red • strong, red • noticeably strong, red • considerably strong, red • somewhat strong, red • lightly strong, red • marginally strong, red • pastel, red • noticeably pastel red • considerably pastel red • somewhat pastel red • etc.

Note that each of the first level of chroma for each name is unmodified. For example, we start with “vivid, red” before using “noticeably vivid, red”. There is only one level for neutral. When a color is neutral, we use the value name with no modification. For example, ‘neutral grey’, or ‘grey’ would mean the same color.

Finally, we need a list of names to specify the various values. • white • medium white • light grey • medium light grey • grey • medium dark grey • dark grey • medium black • black

This give us 28,521 unique color names, however, as Munsell discovered the high chroma colors at the ends of the value range may not be very usable since those colors tend to look similar to each other.

Tint(x) ::= [ x =~ s/[ae]$// ] x .= 'ish' return x; neutral_value ::= [ value_modifier ] , ( value_name | [ grey_modifier ] , grey_name ) ;

chroma ::= [ chroma_modifier ] , chroma_name ;

hue ::= ( [ hue_modifier ] , Tint(hue_name) | null ) , hue_name ;

color ::= [ 'neutral' ] , neutral_value | ( value_tint | null ) , ( chroma | null ) , hue ;

Listing 1. Listing 1 is the Backus Naur Form84,85 description of this color naming schema.

The identifiers hue_name, chroma_name, grey_name, value_name, hue_modifier, chroma_modifier, grey_modifier, and value_modifier define the lists of words that make up the color names.

‘Tint’ defines a function that turns a hue or value into a tint word. For example, Tint(red) gives ‘reddish’, Tint(gray) give ‘grayish’. Strictly speaking, there is no way to define a function in BNF, but I needed one for these definitions.

A color is either a neutral_value, or a value_tint, chroma, and a hue. A neutral_value can optionally have the word ‘neutral’ in front. If no value_tint is provided, ‘grayish’ is assumed. In this ‘grayish’ means the color is in the middle of the value range, as an unmodiﬁed color in HSL would be. If no chroma is provided, ‘vivid’ is assumed.

A hue may be an optional hue_modiﬁer and hue_tint followed by a hue_name. For example, ‘slightly orangish red’ has the hue_modiﬁer ‘slightly’, the hue_tint ‘orangish’, and the hue_name ‘red’.

A chroma can be composed of an optional chroma_modiﬁer, and a chroma_name.

A value_tint can have an optional value_modifier and a value tint, such as ‘medium blackish’. It may also be an optional value_modifier, an optional grey_modifier, and one of the greys. For example, ‘medium light grey’.

A color can be a neutral_value, optionally preceded by the word ‘neutral’. The word ‘neutral’ is optional because the use of one of the values or greys is sufficient to indicate that the specified color is neutral. No chroma_modifiers may be used with neutral, even

84 http://en.wikipedia.org/wiki/Backus–Naur_Form 85 http://en.wikipedia.org/wiki/Extended_Backus–Naur_Form

Because this is a partitive color naming system, there are no mathematical functions that can convert these color names to other color spaces. For example, to convert ‘slightly reddish orange’ to red-green-blue you would need a look up table correlating the name to it’s respective RGB values.

This color naming system is a starting place for the creation of a definitive naming system. As I mentioned above it may be advisable to find other names for some of the hues, such as ‘jaune’, or some of the modifiers, such as ‘mild’. It may also be possible to expand the language to include operations such as ‘complement’, or ‘analogous’. Let’s start a discussion about the system and see where we can take it.

Conclusion We have looked at a brief history of the use of color from the dark to light colors of the Ancient Greeks, through da Vinci’s use of oil colors and introducing new ways to think about color, to modern experiments with light and color using the colorimeter invented by James Maxwell in the mid-1800s. We’ve also looked at the speciﬁcs of the construction of a number of color spaces.

This has been a necessarily brief survey of the history and theory of color. Many books have been written on the subject and it is a very complex subject. While we may have a better understanding today than people three or four hundred years ago may have had of how color works, and its interaction with the human eye (and even the non-human eye), there is still a lot about color that we cannot describe adequately, and don’t understand.

There are many different commonly used color spaces. We’ve also seen that it is possible to create your own color space by picking a set of colors you want to use, or to create a color space mathematically with imaginary colors. While there is no one color space that is best for all applications, if we are familiar with different color systems, it makes it easier for us to pick the color space that will match any work we are doing.

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